There have been devised measures to compensate for gravity (weight of an object) and reduce a load applied upon vertically shifting the object, in addition to basic measures such as a counter weight and a constant force spring (see JP 3794743 B1 and JP 4144021 B1, for example).
However, in the case of adopting the configuration including, as the measure to compensate for gravity, the counter weight, an elastic member such as the constant force spring or a spring, there is required troublesome work such as replacement of a component or modification of the structure in order to deal with variation of load weight applied by an object. Furthermore, force is applied to the elastic member in a state where load weight is applied, in which case it is more difficult to deal with variation of load weight. In a case of compensating for gravity with use of a pneumatic cylinder, variation of load weight can be easily dealt with by controlling the volume of air in the pneumatic cylinder. However, in a conventional configuration, air needs to be charged or discharged every time displacement occurs, resulting in an increase in consumption of air, which is problematic.